This invention relates to intercalated graphite flake having excellent exfoliation characteristics and to a process for preparing it. In particular, the preferred products of the invention have can be exfoliated at low temperature to good volume.
Graphite is a crystalline form of carbon wherein the atoms are bonded in flat layers (basal planes) with weaker bonds between the layers. This structure has enabled the preparation of expandable graphite flake by both chemical and electrochemical intercalation techniques. Once prepared, by either process, the intercalated graphite flake can be expanded, i.e., exfoliated, to greatly increase its volume by heating to a suitably elevated temperature.
Each of the basal planes is comprised of hexagonal arrays or networks of carbon atoms. These basal planes are substantially flat and are oriented or ordered substantially parallel to and equidistant from one another. These basal planes are linked or bonded together and groups thereof are arranged in crystallites. Highly-ordered graphite consists of crystallites of considerable size, with the crystallites being highly aligned or oriented with respect to one another and having well-ordered basal planes.
The graphite structure is typically described as having two axesxe2x80x94the xe2x80x9caxe2x80x9d and xe2x80x9ccxe2x80x9d axes or directions. In this context, the xe2x80x9caxe2x80x9d axis is in a direction parallel to basal planes and the xe2x80x9ccxe2x80x9d axis is in a direction perpendicular to the basal planes and the xe2x80x9caxe2x80x9d axis.
The parallel layers of carbon atoms (basal planes) are held together by van der Waals forces, which are relatively weak. Graphite can be oxidized chemically or electrochemically to appreciably open a space between the basal layers so that a number of different molecules (also known as intercalants) can be entrapped between the layers. The process of intercalation provides a graphite flake capable of marked expansion in the xe2x80x9ccxe2x80x9d direction to form an expanded or intumesced graphite structure in which the laminar character is substantially retained.
At elevated temperature the intercalant decomposes and volatilizes to generate gases, causing the flakes of intercalated graphite to expand in an accordion-like fashion, increasing the dimension in the direction perpendicular to the crystalline planes of the graphite (in the c-direction). The result is the production of particles having a vermicular or worm-like structure.
Intercalated graphite flake has many useful applications. A common application is to exfoliate the intercalated graphite flake into vermicular-like structures which are then compressed into sheets of flexible graphite for use in the manufacture of a flexible foil, e.g., as described in U.S. Pat. No. 3,404,061. The sheets can be cut into desired shapes, or otherwise formed into gaskets, packing materials, fuel cell components, various mechanical and electrical component parts, or the like.
Intercalated graphite flake can also be used in a variety of products that take advantage of the high expansion characteristic of intercalated graphite flake when exposed to high temperature. One such product is a fire-retardant for use in combination with polymer foams to form seat cushions and furniture upholstery in aircraft. Upon exposure to high temperature conditions caused by the combustion of the polymer foam or a nearby material, the heat will cause the particles of intercalated graphite to exfoliate and thereby reduce or prevent combustion of the polymer foam and may, of itself, smother a fire. Since it is important to suppress or retard a fire before it has begun to spread, it would be a substantial advantage for an intercalated graphite flake product to exhibit a very high degree of exfoliation upon exposure to relatively low temperatures and preferably stay ahead of a developing flame or combustion front.
Preparation by the conventional chemical process entails oxidizing particles of graphite, such as natural graphite flake, with an intercalant, eg., a solution of sulfuric and nitric acid, to form a compound of graphite and the intercalant. This process can be accomplished in a bath of suitable intercalant solution. The treated particles of graphite are separated from the bulk of the intercalant, rinsed and dried. The resulting product is referred to as intercalated graphite flake. Representative processing of this type is described in the above-noted U.S. Pat. No. 3,404,061.
Preparation by electrochemical means can comprise contacting graphite flake with an intercalant and passing a current through the intercalant between a cathode and the graphite flake as the anode. Unlike the chemical process described above, the need for a strong chemical oxidant is replaced by the use of the electric current under a suitable voltage. Thus, strong oxidants such as nitric acid, filming nitric acid, permanganate, peroxides, among others, can be dispensed with. Typical of this type of processing are the procedures disclosed by U.S. Pat. No. 4,350,576 to Watanabe, et al., U.S. Pat. No. 5,503,717 to Kang, et al., and U.S. Pat. No. 5,698,088 to Kang, et al., the disclosures of which are incorporated herein by reference to illustrate cell construction and operation.
The first of these, Watanabe, et al., employs a more dilute acid solution as the electrolyte/intercalant than permitted in the chemical process of U.S. Pat. No. 3,404,061. Indeed, Watanabe, et al., find utility in solutions of only 30% sulfuric acid, but preferably employ 50% or more and eliminate the need for concentrated sulfuric acid or fuming nitric acid. However, the description does not address improving expansion, such as by reducing the intumescent temperature or improving the degree or rate of expansion.
In the process of Kang, et al., in U.S. Pat. No. 5,503,717, the use of sulfuric acid is dispensed with and, because natural graphite flakes are electrochemically intercalated with zinc chloride in aqueous solution, the graphite intercalation compound (referred to as a GIC) can be exfoliated thermally without the evolution of SO2 or SO3. However, it is stated in U.S. Pat. No. 5,698,088, that this preparation method is not ideal since the rapid heating of the GIC results in the release of chlorine in particular, which can be deleterious. Again here, the description does not address reducing the intumescent temperature or improving the degree or rate of expansion. And, the reduction of SO2 and SO3 is accompanied by added chloride.
Finally, in U.S. Pat. No. 5,698,088, Kang, et al., eliminate the need for chemicals containing either sulfate or chloride and rely instead on formic acid as the sole intercalant. However, the process appears to take extended reaction times to achieve suitable exfoliation volumes. Indeed, in the examples given, an intercalation reaction time of one hour did not impart any expansion to the flake. As to expansion temperature, they state only that the product may be rapidly expanded. They note that when a graduated quartz glass beaker is heated to a temperature between 400xc2x0 C. and 800xc2x0 C. in a muffle furnace and 0.1 g of product is rapidly poured into the beaker and held for 10 seconds, the measured expansion volume, depending on the chosen reaction time and current density, fills between 150 mL/g and 300 mL/g.
There remains a need for an electrolytic intercalation process capable of efficiently preparing intercalated graphite flake having excellent exfoliation characteristics, in particular, to provide products that can be exfoliated at low temperature to good volume.
It is an object of the invention to provide an improved electrolytic intercalation method capable of producing intercalated graphite flake having excellent expansion properties.
It is another object of the invention to provide an improved electrolytic intercalation method that enables preparing expandable graphite flake exhibiting one or more improved exfoliation characteristics.
It is a specific object of the invention to provide an improved electrolytic intercalation method that enables preparing expandable graphite flake exhibiting reduced exfoliation temperature.
It is another specific object of the invention to provide an improved electrolytic intercalation method that enables preparing expandable graphite flake exhibiting increased expanded volume (also referred to as xe2x80x9cworm volumexe2x80x9d).
It is an object of the preferred form of the invention to provide an improved electrolytic intercalation method that enables preparing expandable graphite flake exhibiting reduced exfoliation temperature and increased expanded volume (also referred to as xe2x80x9cworm volumexe2x80x9d).
These and other objects are achieved by the invention which makes practical use of a discovery that when graphite is electrolytically intercalated in the presence of an expansion aid, the resulting intercalated flake will exhibit one or more areas of improvement with regard to expansion. The invention provides both new methods capable of achieving these objectives and to the novel products of these methods.
The method entails contacting graphite flake with an organic expansion aid either before contact with an aqueous intercalant solution or by dissolving the expansion aid in the aqueous intercalant solution prior to subjecting graphite flake to an electrolytic oxidation treatment therein. The graphite flake is then subjected to electrolytic oxidation to provide intercalated graphite flake. Then, following intercalation, the intercalated graphite flake is recovered from the bulk of the intercalant solution and is preferably washed and further treated with a suitable surfactant in order to reduce the exposed gallery acids on the subsequently dried flake.
Many preferred aspects of the invention are described below.